Electric meter modification and multimeter design

The ammeter head can generally only measure very small currents. If you want to use it to measure larger currents and voltages, you must modify it to expand the range. Various multi-range meters (multi-purpose multimeters) are made in this way. A multimeter is a basic electrical instrument with a wide measurement range, simple structure, and easy use. It is an essential tool for electromagnetic workers. This experiment requires students to master the relevant knowledge first, and then design and assemble a simple multimeter. 

Ammeter modification and calibration
　　In electrical experiments, ammeters (voltmeters and ammeters) are often used for measurement. Commonly used DC ammeters and DC voltmeters have a common part, often called the meter head. The meter head is usually a magnetoelectric microammeter, which only allows microampere current to pass through and can generally only measure very small currents and voltages. If it is used to measure larger currents or voltages, it must be modified to expand its range. The modified microammeter has multiple uses such as measuring larger currents, voltages and resistances. If a rectifier circuit is used in the meter to convert AC into DC, it can also measure related parameters of AC. Almost all the various meters we come into contact with in daily life have been modified, so learning to modify and calibrate the meter is very important in the electrical experiment part.

[Pre-study requirements]
　　1. Understand the wiring method of voltage division and current limiting of the sliding-wire rheostat, and how to adjust the thickness.

　　2. Be familiar with what to pay attention to when measuring the internal resistance of the meter head.

　　3. Master the methods and conditions for expanding the range of electric meters. 
[Purpose of the experiment]
 
　　1. Learn the principles and methods of expanding the range of microammeters and converting them into voltmeters.

　　2. Learn the principles and methods of measuring the full bias current and internal resistance of the microammeter.

　　3. Be familiar with and master the various uses of sliding-wire rheostats.

　　4. Preliminary use of multimeter to check circuit faults.
 
[Experimental instruments]
 
　　　　microammeter, standard ammeter, standard voltmeter (the so-called standard meter is a meter with an accuracy level two levels higher than the given meter head), DC regulated power supply, sliding rheostat, resistance box, single-pole double-throw switch, dry battery, wire
 
[Experimental principle]
 
　1. Modification of the meter

　　1. Measurement of internal resistance of the meter

　 The resistance Rg of the meter coil is called the internal resistance of the meter. There are many ways to measure it. Here we introduce an alternative method. The measurement circuit is shown in Figure 35-1. Set K2 to 2 and adjust R0 so that the meter A0 is at a larger indication (while paying attention that the pointer of meter A does not exceed the range); set K2 to 1, keep R0 unchanged, and adjust Rn so that the meter A0 is at the original position, then Rn =Rg. 
 

　　2. Convert the meter head to an ammeter

　　The microammeter used for modification is called "header". The current Ig required to make the needle deflect to the full scale is called the range. The full-scale current of the head is very small and is only suitable for measuring currents in the microampere or milliampere level. If you want to measure a larger current, you need to expand the current range of the meter. The method is: connect a resistor Rp in parallel at both ends of the head, so that the part of the current that exceeds the head can withstand flows through Rp. The whole composed of the head and Rp is the ammeter, and Rp is called the shunt resistor. By choosing different sizes of RP, you can get an ammeter with different ranges.

　　As shown in Figure 35-2, when the meter is full, the total current passing through the ammeter is I, and the current passing through the meter is Ig.

because 

So  (35-1)

　　The specifications of the meter head Ig and Rg are measured in advance. According to the required ammeter range, the parallel resistance value can be calculated by formula (35-1). Usually, since the ammeter range I is much larger than the meter head range Ig, the parallel resistance Rp is much smaller than the meter head internal resistance Rg.

3. Convert the meter head into a voltmeter

　　The full-scale voltage of the meter head is also very small, generally a few tenths of a volt. In order to measure a larger voltage, a resistor Rs is connected in series to the meter head, as shown in Figure 35-3, so that the voltage that exceeds the meter head can withstand drops on the resistor Rs. The meter head and the series resistor Rs form a voltmeter, and the series resistor Rs is called the extended range resistor. By selecting Rs of different sizes, voltmeters of different ranges can be obtained.

because 

We can get  (35-2)

　　The Ig and Rg of the meter are measured in advance. According to the required voltmeter range, the resistance value to be connected in series can be calculated by formula (35-2). Generally, since the range U of the voltmeter is much larger than the range Ug of the meter, the series resistance RS will be much larger than the internal resistance Rg of the meter.

When the total voltage remains unchanged, if the total resistance of the circuit doubles, the current is reduced to half. Based on this simple principle, an ohmmeter can be made using the meter head. As shown in Figure 35-4: 

　1. Short-circuit the positive (A red test lead) and negative (B black test lead) of the ohmmeter, and adjust the current limiting resistor  so that the meter pointer is full scale. The total current at this time  is or  . The formula  represents the total internal resistance of the ohmmeter, that is, 

Where  is the internal resistance of the meter,  is the shunt resistance,  is the current limiting resistance, and  is the internal resistance of the power supply. If the resistance to be measured is connected between the positive and negative ends of the two test pens (A and B) of the ohmmeter  , the current drops to  , that is,

when 

When,  ;  When,  ;

When,  when,  ;

hour,  .

　　It can be seen that when the measured resistance  is equal to the total internal resistance of the ohmmeter  , the pointer of the meter is at the center of the dial, so  it is also called the center resistance or the median resistance. In theory, any ohmmeter can measure any  resistance (from 0~). But in , its resistance value cannot be read accurately, which leads to a large reading error. In order to get an accurate reading, it can be divided into decimal levels, and each level has its own total internal resistance.

　　For the ohmmeter scale, the scale value is calibrated by the center resistance value of R × 1. This center resistance value is called the center resistance value of the ohmmeter dial. Its relationship with the center resistance value of a certain gear is: the center resistance value of a certain gear = the center resistance value of the dial × the multiplier of the gear. The size of the center resistance value is not only related to the sensitivity of the meter head, but also to the number of power dry batteries and their newness (the size of E).

5. Brief description of multimeter

　　A multimeter is a commonly used instrument. It can measure AC and DC voltage, AC and DC current, and resistance, etc. It has a wide range of uses, but low accuracy. When the selector switch is turned to the DC voltage range, it is a multi-range DC voltmeter. When the selector switch is turned to the DC current range, it is a multi-range ammeter, which is composed of microammeters connected in parallel with ranges of different low resistance values. When used, it must be connected in series in the circuit, and be careful not to connect the positive and negative ends of the test leads in reverse. When using an ohmmeter, before each measurement, connect the test leads A and B (short circuit), adjust the "ohm zero" knob (it is a variable resistor) to make the deflection full, that is, the pointer indicates zero ohms, to ensure the correctness of the scale. And the ohm zero point should be readjusted after each change of range. Note: Do not use an ohmmeter to measure live resistance!

2. Calibration of electric meter

　　After the meter is expanded or modified, it needs to be calibrated. The purpose of calibration is: (1) to assess whether the meter still meets the accuracy level of the original meter after the expansion or modification; (2) to draw a calibration curve so that the meter can be accurately read after the expansion or modification.

The so-called accuracy level is the quality index stipulated by the state for electric meters, which is indicated on the dial of the electric meter in digital form. If it is indicated as  level (  one of the seven levels :
0.1, 0.2, 0.5, 1, 1.5, 2.5, 5, etc.), the maximum indication error of each point is:  ,  .

The commonly used and simple calibration method is the comparison method, which compares the table to be calibrated with a higher-level standard table.

　(1) The ammeter with expanded range can be calibrated with a standard ammeter. The circuit is shown in Figure 35-5. The calibration points should be selected at the positions of the scale values ​​within the full deflection range of the ammeter after the expanded range, and  the values ​​of each calibration point should be determined. This can not only be compared with the grade error  to determine whether each calibration point  exceeds  the value , but also a curve can be drawn  for correction of the readings when used.

　(2) The modified voltmeter is calibrated with a standard voltmeter. The circuit is shown in Figure 35-6. The calibration points should be selected at the positions of all scale values ​​of the modified meter. The values ​​of each calibration point are determined  and a curve can be  drawn.

　(3) Calibration of electric meter

The calibration results of electric meters are usually expressed as calibration curves in addition to grades.